The Evaporator Fan VFD Initiative

Introduction

During the last five years, a number of controlled
atmosphere storage facilities in the Pacific Northwest have
installed variable frequency drive (VFD) technology on
evaporator fans. VFD in the application have the potential
for very large energy savings during the low refrigeration
loads of the holding season. Despite this potential, VFD
installations in this application have been very limited
due to concern regarding the impact of reduced air flow on
fruit quality. Despite scattered qualitative reports of
"good or better fruit quality" from those that
have implemented the technology, no truly scientific
evidence has been gathered.

This paper introduces the Evaporator Fan VFD
Initiative, a research and market transformation program
sponsored by the Northwest Energy Efficiency Alliance
("The Alliance"). Through a focused program of
field trials, fruit quality assessment, and education, the
Alliance hopes to prove the value of VFD in the
refrigerated warehouse market, with significant effort in
the controlled atmosphere storage sector.

Cascade Energy Engineering has been selected as the
contractor to manage and implement this market
transformation program.

Northwest Energy Efficiency Alliance
The Northwest Energy Efficiency Alliance is a
non-profit consortium of utilities, governments,
public-interest groups, and the private sector in the
northwest (Washington, Oregon, Montana and Idaho). Over the
next three years, the Alliance has committed approximately
$65 million toward various energy efficiency market
transformation projects in the region. The goals of a
market transformation project are to affect the long-term
industry standard patterns for design, purchase and
operation of electrical energy consuming equipment.

Toward this goal, the Evaporator Fan VFD Initiative
is focused on answering some fundamental fruit quality
questions that over-shadow the potential energy savings for
this application. With an estimated 80,000 hp in
refrigerated warehouse evaporator fans, the potential
regional energy savings is enormous.

VFD Basics

All 3-phase induction motors are designed to operate
at one speed; typically 1200, 1800 or 3600 rpm. Since the
electric utility provides power at 60 cycles per second
(Hertz), motor speed is determined by winding
configurations within the motor. As a result, equipment
driven by induction motors (including fans and compressors)
are constrained to a single speed.

A VFD allows nearly any induction motor to vary its
speed. The 60 Hertz sine-wave from the utility is converted
to DC power. Modern pulse-width modulated VFDs then send
pulses of varying width and polarity to the motor. Although
the voltage waveform no longer resembles the original
sine-wave, the current draw of the motor is remarkably
sinusoidal. The result is a motor that can now be operated
at any speed between 0 and 60 Hertz.

Fan Savings with VFD Control

Although VFDs can produce energy savings on many different equipment loads, evaporator fans offer some of the greatest potential savings. This is the result of the affinity laws, which govern the flow, pressure, and power requirements of a fan or pump in a closed, un-damped system:

Air Flow - Fan Speed

Pressure increase - Fan Speed2

Power consumed - Fan Speed3

Evaporator fans in a refrigerated warehouse are a classic example of these laws that offer nearly perfect cubic power reduction with speed.

This means that a fan operating at 50% speed will ideally draw 12.5% shaft power. Although motor and drive efficiency might temper this ideal value to the range of 15%, the power reduction is tremendous. And to top off the fan energy reduction, any savings in fan energy results in reduced room heat loads. Compressors unload and condenser fans operate less as a result, adding another 20% to 30% to the direct fan energy savings.

Typical Installation Cost & Payback

A wide range of VFD electrical installations have been tried for evaporator fan applications. Through a number of established projects, it appears that a reasonable budgeting cost of $400/hp for VFD installation can be used. Thus, a sample room with (5) 2 hp motors should cost roughly $4,000 to retrofit to VFD control.

Ultimately, final economics depend on a number of variables, including hours of operation, current operating strategies (constant or cycling), fan horsepower and utility rate schedules. In regions with a higher energy cost (e.g., Yakima & Medford), the payback can be a few years. In regions with a lower energy rate (e.g., Wenatchee & Hood River), the payback can be as high as 8 to 20 years. As discussed later in this paper, without an improvement in fruit quality or utility incentives, regions of low energy cost may not realize acceptable economics from evaporator fan VFD installations.

Current Users

A number of prominent companies have begun experimenting with VFD technology on evaporator coils. These include:

Bear Creek (Harry & David)

Broetje

Clasen

Duckwall-Pooley

Naumes

Stadelmans

Washington Fruit

For all of these companies, the energy saving was dramatic.

Limited fruit quality assessments are also encouraging: Broetje personnel weighed bins of fruit before and after storage and compared data for evaporator fan VFDs with fan cycling. They reported reduced mass loss with the evaporator fan VFDs.

An analysis of pressure test readings after storage at Washington Fruit showed higher pressures in four of five rooms relative to similar fruit in other rooms where fan operation was continuous.

Simple Field Trials

Fruit quality research will encompass a several
techniques of varying degrees of complexity. These tests
can be divided into simple and detailed field trials.

Over the next two harvest seasons, fifteen simple
field trials will be conducted. These tests will compare
fruit mass loss over the course of a full holding
season.

Participants will commit two CA rooms to the trial.
VFDs will be installed in one room and a second room will
be used as a "control" room for comparison. To
the degree possible, the two rooms should be identical in
terms of size, bin capacity, bin stacking pattern,
evaporator model, fruit variety, and storage schedule. The
rooms should also be filled within a day or two of each
other.

As the rooms are filled, Cascade Energy Engineering
will divert a bin of fruit from which small sample bags of
fruit will be created for both rooms. Samples will be
weighed prior to storage. Other basic fruit quality tests
will also be conducted. These sample bags will then be
placed in multiple locations in the stack as the room is
loaded. Samples will be placed in both the VFD room and the
control room.

When the rooms are opened for packing, Cascade will
retrieve, weigh, and evaluate samples to determine the
impact of the VFD operation on fruit quality.

Detailed Field Trials

Three more detailed field trials are planned that will investigate the influence of reduced fan speed on air velocities, temperature distribution, and humidity in the space. As in the simple field trials, this research will focus on comparing two identical rooms.

Other Field Trials

Another series of evaporator fan VFD field trials are geared toward non-CA applications. These tests could take place in common storage rooms or in refrigerated warehouses in other industries.

There will also be co-funding for two full scale evaporator fan VFD installations in the region. Participants for the full scale installations will be selected from those who participate in these early field trials.

Drive Installation

Following review and assessment of candidate facilities and rooms, test sites and rooms will be selected. An electrical contractor will be employed to install the drive along with any necessary bypass and motor protection features. In addition, the vendor of the existing computer control system would be employed to modify the hardware and algorithms for speed control.

Project Cost Sharing

The evaporator fan and motor protection costs will be shared between the Northwest Energy Efficiency Alliance and the warehouse owner. All other costs, including data acquisition and fruit quality assessment are borne by the Cascade Energy Engineering, and ultimately by the Alliance.

Plans for the 1998-1999 Season

A total of twelve field trials (10 simple and 2 detailed) will be conducted during the 1998-99 storage season. Assessment of fruit quality will begin with the first room opened, likely in the spring of 1999. These results will be documented and shared with the owners, operators and contractors of the fruit storage industry. If possible, some results may be presented at the 1999 Post-Harvest Convention. Ultimately, all results will be available for review by the entire fruit industry.

Marcus H. Wilcox, P.E. President and Robert D. Morton P.E., Vice President